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Amphibians Essay, Research Paper

AmphibiansIntroductionAmphibians
are unique life forms in that there is no single characteristic that
describes amphibians, such as hair on mammals and gills on fish. The
name, Amphibia means “double life” (Duellman 1). The term
Amphibian can be interpreted in two ways, either as an animal
spending part of its life in water and then changing to an aquatic
adult, or as an animal spending its life in and out of water.
Amphibians are surprisingly variable. There are three main types of
Amphibians (Cochran 8). They are caecilians, salamanders, and
anurans. Caecilians are legless, wormlike amphibians (Vogel 79). They
normally live in burrows of tropical regions. All salamanders have
scaleless skin, four legs and a tail. Anurans are either frogs or
toads. They normally move around in hops. Many amphibians
metamorphose, or change, from a gill breathing aquatic larvae to an
air breathing terrestrial adults. These amphibians have great
abilities. A new born tadpole carries an aquatic life. After it
metamorphoses, it becomes terrestrial. It is like being born again.
The tadpole learns to travel by means of jumps, as well as to swim
with limbs. This variability of amphibians make them one of the most
interesting species to study and indeed amphibians have contributed a
great deal to science. Studies on amphibian metamorphosis provide
scientists with knowledge about the actions of the thyroid and
pituitary hormones. Because of the ease of their breeding in
laboratories and their relatively simple chromosome complements,
important advances in studies of hybridization and speciation have
been made. The study of vocals of frogs have provided advances in
animal sound communication. The poison from poison-dart frogs shows
great medical promise, especially for patients who do not respond
well to pain killers made from opium seeds. The poison is far
stronger and is two-hundred times more powerful than morphine, in
killing pain. Evolution Of AmphibiansAmphibians are the oldest known
land vertebrates and accordingly have been ideal subjects to study
the process of evolution. The earliest amphibians existed 360 million
years ago (Radinsky 87). The early amphibians known as
ichthyostegids, had a tail fin and canals on the skull. These two
features suggest that they spent most of their time in the water.
Between 340 and 250 million years ago, the population of amphibians
grew, and several groups emerged. One group of amphibians, known as
labyrinthodonts, grew to extraordinary size. They could be up to four
feet long. They were short-legged and large headed. Their skulls were
deep and massive, and their jaws were lined with small, sharp,
conical teeth. Also, there was a second row of teeth on the roof of
the mouth. Many labyrinthodonts had a notch on the back of the skull
which acted as a large eardrum and it transmitted vibrations to the
inner ear. Their bulky skeleton and their short limbs suggest that
the majority of the labyrinthodonts were slow, clumsy walkers on
land, and they probably spent most of their time in shallow water.
The large jaw and sharp teeth suggest that they were predators and
they fed on large prey. As some labrythodonts strengthened their
limbs as adaptations for life on land, they reduced the size and
strength of their limbs. The weaker ones developed long, flexible
bodies with weak vertable columns. Scientist interpret this to mean
that they had secondarily adapted to full aquatic life. Their heads
flatted which suggested that they spent much time lying on the floor
of a lake or pond. Some developed long and narrow snouts, which was a
good characteristic to have for traveling through water. Another
grouping of early amphibians were the lepospondyls. They were mostly
small, less than a foot, with long, slender bodies and weak legs. The
lepospondyls lacked the notch on the back of the skull. They had
small sharp teeth which suggested that they fed on small
invertebrates. A later group, known as the nectrideans evolved more
long bodies. Their weaker limbs, and less stiffened vertable columns
suggested that they had almost become completely aquatic. Like the
labrinthodonts, the nectrideans had the notch on the back of there
skull which acted as an ear drum. A last group called the
diplocaulids, developed large, flattened, triangular skulls, with the
dorsal eyes and nostrils that are normally a characteristic of
animals that live on the floor of a body of water. The study of these
early amphibians provide convincing evidence of evolution.
ReproductionMost amphibians go through annual periods of mating
(Duellman 19). In anurans, rainfall can cause mating. Amphibians
reproduce sexually. Some amphibians lay their eggs and let them
develop by themselves. Others, lay eggs and guard them. The mother is
usually the one who guards the eggs. A salamander will guard her eggs
by wrapping herself around them. Some anurans carry their eggs
imbedded in their backs. Eggs and LarvaeAll amphibian eggs are
basically the same in that there are layers of semipermeable
membranes surrounding the ovum. However, a lot of differences exist
in individual eggs, such as size. Eggs laid in water form into large
clumps, or they are scattered and deposited at different sites.
Normally, the clumps are attached to sticks or vegetation in the
water. This serves to maintain the position of the clutch. The
terrestrial eggs are sometimes in strands connected by jelly between
each of them. They can also be stacked on top of each other in a
pile. Most anurans sort their eggs in piles and form nest around
them. Amphibians embryos contain all the nutrients for their
development until hatching. Even if the mother bears live young, all
of the nutrients for embryonic development are provided by the yolk,
not the maternal tissues. Amphibian embryos normally get oxygen from
external gills. In salamanders, three pairs of external gills provide
for oxygen in take. Amphibian embryos generally remove waste in the
form of ammonia. The majority of amphibian larvae are aquatic. Unlike
the embryos, amphibian larvae obtain nutrients from the environment
for development and growth. Most larvae feed on small aquatic
invertebrates. Some amphibians eat algae and continue to feed and
grow until there is no more algae left. Larvae growth rates are
dependent mostly on temperature and food availability. Some chemicals
left by previous larvae can slow growth down. Anurans tadpoles are
mostly short and develop length through growth, in salamanders the
larvae are long. Most species of salamander and anurans stay in
schools as larvae. They stay in schools to avoid predators and to
have bigger food supply. An organism that is slightly bigger than the
larvae could not attack a school. Some species of tadpoles stir up
the bottom, releasing mixtures of particles of food. MetamorphosisA
metamorphosis is a series of complex physiological, biochemical, and
behavioral transformations. Three major kinds of changes occur during
an amphibian s metamorphosis (Vogel 83). The first one is the removal
of structures and functions that are significant only to the larvae.
The second kind is transformation of larval structures into a form
suitable for adult use. The last kind is development of new
structures and functions that are essential to the adult. There are
many fundamental as well as subtle changes throughout the changes of
metamorphosis. For example, during metamorphosis, larger larval red
blood cells are replaced with by smaller adult blood cells. Also,
during metamorphosis, the amphibian gills are completely removed. In
anurans, the intestine reduces in its size during metamorphosis. Even
the vitamins in the eye change. In larvae, the eye contains vitamin
A2, while the adults have vitamin A1 in the eye. Perhaps, the
greatest changes occur in the respiratory mechanisms. Lungs are
formed when the flow of amino acids and thymidine into the lung
tissues is increased. In the gill tissues, the flow of amino acids
and thymidine is decreased causing them to eventually disappear. Some
of the more obvious changes are the removal of the tail and
development of eyelids. Initially, the fins reduce in size. Then the
tail becomes smaller and smaller until it is removed. The formation
of the mouth is very important to an amphibian. During the
development of the mouth and jaw, the animal is unable to feed. In
preparation for this phase, large quantities of foods are stored.
Metamorphosis is a fascinating transformation, providing scientists
with a greater understanding of the biochemical and physiological
processes. Relationships to the EnvironmentMost amphibians avoid
daytime temperatures and low humidity. During the day, they usually
stay in areas with high moisture content, and they stay in insulated
areas away from air currents. Inside a log and in mounds of soils
serve as good places to spend the day. Amphibians may come out during
the day, but only if there is a sufficient amount of moisture.
However, they might risk water loss or even death to accomplish some
goal such as feeding or mating. Amphibians can reduce water loss by
reducing the amount of surface area exposed to evaporation. Some
salamanders coil their bodies tightly to prevent evaporative water
loss. Tree frogs reduce surface area by selecting a shaded site and
tucking limbs close to the body. Some amphibians dig deep burrows and
stay there for up to nine months at a time. Terrestrial amphibians
are generally nocturnal, with the exception of some species of
anurans. The skin of amphibians is highly permeable. Most salamanders
and all frogs that live in aquatic areas have smooth skin on the
belly and sides. Most terrestrial anurans have rough skin on the
belly and the thighs. The rough stomach surfaces provide a great
surface for water absorption. Even here, the amphibians have proven
to be capable of “double life”. For example, sometimes, the
amphibian may not want water to enter the body. For such occasions,
some amphibians have developed ways of waterproofing the skin. They
form a cocoon that encases the body. They make these cocoons during
long periods of dormancy and during the day. An example would be the
salamander Siren intermedia. Siren intermedia burrows into the mud at
the bottom of drying ponds (Duellman 198). They make the cocoons to
prevent too much moisture from entering the body. In frogs, the
cocoon is made of a dry substance called statum corneum. It encases
the entire frog leaving openings in the nostrils.

Amphibians
generally have temperatures close to that of their immediate
surroundings, and are therefore, categorized as cold blooded.
Amphibians are not capable of internal heat-production so the body
temperature and environment temperature are about the same.
Therefore, amphibians are tolerant to a wide range of temperature.
For example, some Central American salamanders can stand temperature
between -2.0|C and 30|C. Some anurans can stand temperatures between
3.0|C and 35.7|C. Also, to raise body temperature, some anurans lay
in the sun; however, this could create water loss problems, requiring
them to balance their needs. Food and FeedingThe feeding strategies
of amphibians include their choice of prey and the ways they locate,
capture, and eat the prey. All adult amphibians are carnivores. They
feed mostly on insects and, but some eat a wide variety of
invertebrates. Some large anurans, such as Ceratophrys ornata feed on
large prey, such as birds, turtles, snakes, and other anurans (Vogel
83). As amphibians grow larger, the kinds of prey they select may
change. As larvae develop teeth, they capture larger prey. Hylid
frogs eat increasingly larger preys, even during postmetamorphosis.
Seasonal difference in diets have been reported for various species
of amphibians (http://www.syspac.com/ varney/aha.html). The diet of
anurans living in West Africa vary greatly throughout the year. Among
the thirteen species of anurans living in Amazonian Peru, the
difference in food was greatest during the dry season. These
differences are indications of the availability of prey. Some animals
are dormant during the dry season. In some amphibians, the selection
of larger prey is likely when moisture conditions are heavy.
Amphibians vary considerably in their hunting practices. The vast
majority of anurans and salamanders use vision to hunt prey. For the
species that have developed the sit-and-wait strategy, vision is
important. Once a prey is sighted, it may be followed for a short
distance any then captured. Sight is also important in identifying
kinds of prey, such as those with a large energy content and or that
may be distasteful or harmful. For example, toads learned to reject
bumblebees by sight alone. Amphibians also use smell and hearing to
hunt prey. Some species of toads can locate prey just by smell alone.
Smell sense is of great value in tracking the prey, once it is
located. Some amphibians can also detect insects by the sounds they
make. A species of toad, Bufo marinus, is attracted to calling
insects. Amphibians also show major differences in ways of capturing
and in taking the food; however, all terrestrial amphibians except
caecilians use the tongue in capturing prey. Many caecilians and
large anurans use fang like teeth to hold struggling prey. The
tongues of amphibians have glands that produce a sticky substance,
that immobilizes the prey. Terrestrial caecilians feed primarily on
long prey, such as earthworms located on the ground or in burrows.
Prey capture involves a slow approach towards the prey until contact
is almost made, then the prey is captured by a powerful bite. In
terrestrial salamanders, the tongue plays an important role in prey
capture. The salamander s tongue has a sticky substance produced by a
gland. The salamander sticks out its tongue and the prey gets stuck
to it. Then the salamander pulls the tongue in the mouth. The tongue
can reach up to eight percent of the length its body. The entire
capture a prey lasts 0.10-0.15 seconds. Anurans flip their tongues at
their prey. Like salamander s tongue, it is has a sticky substance
that is produced by a gland. The prey gets stuck to the tongue and
then the anuran pulls the tongue back into the mouth. The process is
almost exactly the same as the salamander. The completely aquatic
frogs, known as pipids, do not have tongues. Therefore, they have an
entirely different means catching their food. They suck in food and
water, the water leaves the mouth before it closes completely.
Enemies and DefenseAmphibians are no different than any other animal
because they can be harmed by a wide variety of predators, parasites,
and diseases. Most caecilians, some salamanders, and some anurans are
known to be near the bottom of the food chain (Cochran 11).
Amphibians are also subject to many diseases. Some diseases are
include forms a tuberculosis and cancer. Amphibians can be harmed by
many parasites. While, most of the time parasites live without harm.
Massive infections over an amphibian population has been known to
cause great disaster. For example, a parasite called Pleistophora
which is normally a parasite of fish, caused a lethal epidemic of the
toad Bufo bufo in southern England (Cochran 56). Infestations of the
parasite, Carchesium have been known to clog the gills of tadpoles,
causing retardation and death. Amphibians are prey for a great
variety of predators because they are small and they have soft skin.
Because of their “double life”, amphibians encounter predators
both in water and on land. The predators include all classes of
vertebrates and some arthropods. Some small anurans are even prey to
the plant specie, Venus flytrap. Aquatic eggs of amphibians are
mostly the prey of fish and aquatic invertebrates. The leech is the
most common invertebrate predator of the eggs. Some salamanders feed
on each others eggs. Larval and adult newts also feed on eggs of some
species of anurans and salamanders. Terrestrial eggs are eaten by a
some groups of anurans and a variety of insects and vertebrates. They
include the spider, cricket, crabs, and snakes. The snake from the
genus Leptodeira can shape their jaw to part of a clutch of eggs and
devour the rest of clutch because the eggs stick together. Generally,
amphibians have been classified as defenseless creatures (Duellman
244). However, they have evolved some features which will provide
some protection. Amphibians have a variety of encounter behaviors.
One example of a characteristic that has been evolved is escape
behavior. Escape behavior is when a prey senses the presence of a
predator and then attempt to leave the area. A terrestrial caecilian
will dig into the soil quickly when it senses a predator. When an
aquatic caecilian meets a predator, it will spit a small blob of
water. Also, it will produce large quantities of mucus, which make it
very difficult to hold. Some caecilians are capable of inflicting
painful bites and some have poisonous secretions. A salamander will
get into the position known as the Unken reflex. This is a immobile
posture when the chin and tail are elevated. Toxic skin secretions
come from the glands and cover the salamander. Salamanders also lash
their tails at a predator. Toxic secretions also cover the tail. Some
salamanders also head-butt their predators. They flex the head
downward and lunge at the predator. Most anurans seem to rely on
escape behavior to avoid predators, but some frogs are extremely
poisonous. Many species of amphibians have color patterns that will
match the environment they live in, thus providing a camouflageAn
Interesting AmphibianOne of the most interesting amphibian is from
the anurans group. Phyllobates terribilis, a specie of poison-dart
frog, produces one of the most toxic non-protein substances. The
poison is so strong that it can be lethal to touch. Only 55 out of
every 135 species of poison-dart frogs are known to be toxic. In
addition, the frogs have neon colored skin which warns the predators
to stay away. In the Amazon basin, the natives use the poison-dart
frogs to poison their blow gun darts, which they use for their
hunting for food. To poison their darts, they rub the dart against
the frog s skin (Moffet 98). The poison is effective for more than a
year. Poison-dart frogs range from a half inch to three inches, in
size. They are found in a small area of lowland rain forest in
western Columbia, where they contribute to human beings survival.
ConclusionDue to their great variability, amphibians have provided
and continue to provide scientists with extraordinary opportunities
to study and increase their knowledge. Specifically, amphibians have
contributed to our understanding of the evolutionary process,
particularly as to how physical features and behavior patterns can be
altered to adapt to changing and diverse environments.

505

Cochran, Doris.
Living Amphibians of the World. New York: Doubleday & Company(a)
Inc., 1962Duellman, William. Biology of Amphibians. Maryland: The
John Hopkins University Press, 1994. Moffett, Mark. “Poison-Dart
Frogs.” National Geographic May 1995: Vol. 187Radinsky, Leonard.
The Evolution of Vertebrate Design. Chicago: The University of
Chicago Press, 1987Vogel, Zdenek. Reptiles and Amphibians. New York:
The Viking Press, 1964″Amphibians” http://www.syspac.com/
varney/aha.html

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